X-ray apparatus



June 9, 1925'. 1,541,627

W. D. COOLIDGE X-RAY APPARATUS Original Filed July 9. 1920 Fig. I. {2

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lm/efitor wnnm D. oooud e. b His Attorney.

Patented June 9, 1925.

UNITED STATES PATENT OFFICE.

WILLIAM D. COOLIDGE, OF SCHENECTADY, NEW YORK, ASSIGNOB TO GENERAL ELEC-v TRIO COMPANY, A COBPGBATION 01' NEW YORK.

X-RAY APPARAT S.

Original application flied July 8, 1920, Serial No. 895,088. Divided and this application filed April 28,

1923. Serial No. 884,864.

To all whom it may concern.

Be it known that I, WILLIAM D. Coomnon, a citizen of the United States, residing at Schenectady, in the county of Schenectady,

6 State of New York, have invented certain new and useful Improvements in X-ltay Apparatus (division of my prior application Serial No. 395,088, filed July 9, 1920), of which the following is a specification.

The present invention comprises an X-ray apparatus which is particularly suited for althougi it is not limited in rent, and for a iven subject, an X-ray tube operati by e ectron conduction substantially in ependently of gaseous ionization,

will always produce a radiograph of the same densit for a iven exposure. As explained by rof. J. Shearer in the American Journal of Roentgenology, Vol. 3 (1916 page 300, the radiographic efiect may e expressed as follows in terms of current (I), time (T), voltage (V), and distance ITxV' The distance between the X-ray source and the photographic plate or film can readily be maintained constant.

It would. appear in view of this that with,

a given patient, a doctor or dentist could always get the same sort of radlograph by the use of a simple time switch. As a matter of fact, this ideal condition is obtainable only in a laboratory under carefull controlled conditions. Ordinarily the avai able current supply is extremely variable in voltage, and consequently a time switch alone is insufiicient to ive a substantially constant radiographic e set.

In accordance with my invention, I have provided an X-ra apparatus constructed to give a substantia ly constant radiograplnc effect even with a variable supply of voltage. The novel structural features of my invention will be ointed out with particularity in the appendedclaims.

My invention will be described in detail in the following specification taken in connection with the accompanying drawings in which Fig. 1 illustrates diagrammatically a system embodying m invention; Fig. 2 is adiagram of a set 0 curves showing the currents in the transformer furnishing current for heating the cathode of the X-ray tube; and Fig. 3 illustratesdiagrammaticallyone form of cathode .heatlng transformer suitable for use in my nnproved apparatus,

The system shown in Fig. 1 contains an X-ray-tube l of the hot cathode type supplied with heating currents from a s ecial transforming apparatus 3 which is designed to give a heating efi'ect which varies inversely with the supply voltage. In the con struction illustrated in 'Fi 1 and Fig. 3, the transformer 3 is provi ed with two primary windings 4 and 5. v

' As illustrated in Fig. 3, the primary windings 4 and. 5 are located on separate cores 6 and 7. The core 6 is rectan lar and is so constructed as to provide a ighly saturated magnetic circuit for its primary winding 4 and the secondary winding 8 with normal voltage on the apparatus. The arrangement of the core and windings illustrated provides for an appreciable magnetic leakage between the windin The section 5 of the primary winding 1s wound on an open magnetic core 7 with an auxiliary sec ondary winding.9. The core 7 is so constructed as to operate far below saturation. The'change of voltage at the terminals of the auxiliary secondary winding 9 takes place at a fairly low rate as the primary voltage changes. The core 7 has been shown as an open magnetic core to illustrate this feature. The two primary transformer windings 4 and 5 are connected in series with each other by conductors 10 and 11 to the source of energy represented'by the mains 12 and 13. The main secondary winding 8 and the auxiliary secondary winding 9 are also connected in series, but are so connected that the induced electromotive forcesoppose each other. The conductors 14 and-15 leadingfrom the terminals of these windings are connected in series with an adjustable resistance 16 (Fig. 1), to the primary of a transformer 17, the secondary of which is connected by conductors 18, 19, to the terminals of the cathode 2 of the X-ray tube. In some cases the transformer 17 may be omitted.

Ihe sum of the magnetic fluxes in the saturated and unsaturated magnetic circuit varies in proportion to the voltage variation of the supply circuit 12, 13, but the fluxes in the two secondary windings vary at different rates. By properly propor'tioning the secondary windings, which are connected in opposition, the sum of the voltage in the conductors 14, 15 may be made to decrease as the voltage impressed on the primary windings 4, 5 increases, in such meas ure as to roduce a substantial y constant radiographlceffect.

The operation of the filament lighting transformer will be more clearly understood from the diagram of electromotive forces shown in Fi 2. The voltages induced in the main an auxiliary secondary windings are measured along the vertical line 29. and the currents in the primary windings are measured along the horizontal line 23. Since the core 6 is highly saturated at normal voltages the total electromotive force induced in the main secondary winding 8 varies with the variations of primary current in the general manner represented by the curve 24, 25. A change of the electromotive force in the auxiliary secondary winding 9 is indicated by the curve 26. This curve 26 is drawn below the horizontal zero line to indicate that this elcct-romotive force opposes the electromotive force of the curve 24, 25, the main and auxiliary secondaries 8, 9 being connected in opposition. The resultant secondary voltages across the conductors 14 and 15 therefore may be represented by the curve 27, 28. This resultant curve rises rapidly at first over the portion 27, and then bends downwardly. As the current in the primary winding increases with an increase of impressed elcctromotive force, the voltage in the conductors 14- and 15 decreases.

The cathode 2 and anode 20 of the X-ray tube are connected by conductors 63, (ii to a high potential transformer 65, the primary of which is connected by conductors (K5, (ST to the mains 12, 13. In series with the primary of the transformer is atime switch 68, which has been diagrammatically shown in the drawings, and may be of any preferred construction. When the time switch is set to give an exposure of given length, should the voltage in the main conductors 12, 13 vary, for example, should it increase, thereby increasing the voltage across the terminals of the X-ray tube, and consequently increasing the penetrability of the X-rays, then by the action of the described construction in the filament lighting transformer, the temperature of the cathode decreases at such rate that the resulting radio aphic effect for the given exposure is su stantially constant over a working range.

Conversely, a decrease of voltage of the supply source will result in an increase of V0 tage of the filament lighting current, increasing the temperature of the cathode and thereby increasing the milliamperage of the current through the tube, and thus compensating for the decrease of impressed voltage so as to give over a working range a substantially constant radiographic efl'ect. Instead of varying the current through the tube the other member of the current-time factor may be varied, while the current is maintained constant. This modification is claimed in my prior application of which the present application is a division.

In the system herein described the time interval and the distance from tube to film are maintained constant. When the impressed volt-age varies the space current through the tube, over a working range, will vary inversely substantially as the square of the voltage impressed across the tube. As indicated by the equation given above,

this variation of current maintains the radiographic efi'ect substantially constant. As a slight change in cathode heating current will produce a relatively great change in electron emission and as this change of emission will vary with the tube conditions, the required voltage variation of cathode heating current for a given set of conditions to produce the current variation required to give a constant radiographic efl'ect must be determined by trial and the transformer ratios adjusted accordingly.

It is customary to operate the cathode of an X-ray tube of the Coolidge type at the saturation value of electron en'nssion. In other words, the impressed voltage is sufliciently high to carry away substantially all of the electrons emitted by the cathode. As the electron emission with varying cathode temperature varies at a high rate according to the well-known Richardson equation 0 I=aT e (I being electron current, '1

cathode temperature, a and I) constants and e a logarithmic base), the'size of the transformer core, ampere turns and other transformer characteristics can be chosen in accordance with the well-known principles of transformer design to cause the electron emission to vary inversely with square of the supply voltage. The space current value necessarily must vary at the same rate when the cathode is operated at saturation value of electron emission.

What I claim as new and desire to secure by Letters Patent of the United States, is

1. The method of obtaining a predetermined output of X-rays from an X-ray tube which is energized by a current of variable voltage which consists in regulating the electron emission at the cathode substantially inversely with the square of the voltage.

2. The method of obtaining an X-ray output of redetermined radiographic effect from an -ray device energized b a source of current of variable volta e w ich con sists in timing the interval 0 operation of said X-ray device and regulating the current transmitted by said device substantially inversely with departure of voltage from a predetermined value to produce a constant radiographic effect.

3. An apparatus for operating an X-ra tube with current of variable vo tage whic consists of a main transformer adapted for connection to said tube and constructed to deliver current having a voltage proportional to the voltage of said source, and means for regulating the electron emission of the cathode of said tube inversely with respect to voltage variations of operatin current supplied to said transformer in suc measure as to produce a substantially constant radio raphic effect.

4. An )ray apparatus adapted for o eration from a source of current of variab e voltage comprising an X-ray device. electric connections for operating said devlce from said source, a time switch in circuit with said device, and means for regulating the current in said device substantially inversely in response to voltage variations of operating current, thereb producing an X- ray output of substantia ly constant radiographic effect for a given time nterval of operation.

5. An X-ray apparatus comprising an X ray tube, a source of current subject to variations of voltage connected to nerate X- rays in said tube, means for eating the cathode of said tube by a current varying substantially inversely in voltage with the current which generates X-rays.

6. An X-ray apparatus adapted for operation from a source of current sub'ect to volta e variations comprisin an -ray tube raving a cat ode adapt to operate at incandescence, a transformer connected to generate X-rays in said tube and adapted to be supplied by said source, and means for heating the cathode of said tube b current varying in voltage substantial inverselv with the 0 rating vol 0 said transformer in suc measure as to produce an X-ray output of substantially constant radioiraphic efiect.

X-ray apparatus comprising an to generate X-rays in said tube, an auxiliary transformer connected to heat the cathode of said tube, and a common su ply circuit carrying current of variab e voltage, said auxiliary transformer being constructed to deliver current var in inversely in voltage with variations 0 V0 tage of said supfly circuit.

8. An -ray apparatus comprising an X-ray tube, an energizing source 0 ratively connected to generate X-rays ti erein, a transforming apparatus comprising a lurality of primary windings connect in series to said source, corresponding secondary windings connected in opposition, a core for one of said sets of rimar and secondar windings constructe to be ighly saturate a second core for another set. of said windings constructed to'operate below saturation, said cores bein constructed to have their fluxes vary at Ifi'erent rates in proport on to volta variations in the primary windings, an said cores and secondary windings being proportioned to deliver a current varying in voltage inversely in proportion to the voltage of said source, and electric connections between said secondary windings and the cathode of said X-ray tube.

9. in X-ray. apparatus comprising the combination of a source of alternating current subject to variations of potential, an

'X-ray tube having a cathode adapted to be heated to incandescence by passe of current, means for operating said tu by current derived from said source, and transformlng means connected to said cathode comprising primary windin connected in series, separate cores there or and corre sponding secondary windings connected in opposition arranged and constructed to dehver a heating current for said cathode varym Ill voltage in such measure, as to vary t e cathode temperature in response to voltage variations 0 said source of alternating current, that the radi phic effect of X-rays generated by sai tube for p a given time interval is substantially constant when the supply voltage varies.

10. An X-ray apparatus adapted to beoperated with current of variable voltage comprising an X-ray device, electric connections for operating said device from said source and means for regulating the current in said device substantia 1y inversel in nae to the square of voltage varia ions o operating current. I

In witness whereof, I have hereunto set my hand this 25th day of April, 1923.

WILLIAM D. OOOLIDGE.

X-ray tube, a main transformer connected 

